1. Field of the Invention
The invention relates to thermoplastic containers such as bottles, and more particularly, to such containers with outer layers of a dissimilar material so as to provide improved properties to such container. The invention also relates to a process for preparing such containers.
2. Brief Description of the Prior Art
Blow molded, plastic bottles such as those made from polyethylene terephthalate have largely replaced heavier glass bottles previously used for carbonated beverages and the like. One disadvantage associated with plastic bottles is the extremely thin wall construction of the body of the bottle. The bottles are inherently weak which prevents them from being returned to the bottler and refilled.
In an attempt to overcome the above-mentioned disadvantages, the plastic bottle industry has been manufacturing plastic bottles of a one-piece construction with thicker body walls. This makes the bottles stronger enabling them to be returned to the bottler for refilling.
However, in manufacturing bottles of a one-piece construction with thicker walls, it has been found that such bottles have a serious stress cracking problem associated with them. Stress cracking is the development of fine cracks which are the result of the release of stresses introduced into the bottle during the molding process. Such bottles are manufactured by the blow molding process which stresses the more rigid side walls of the bottles. The stress is particularly in the base area of the bottles which is complexly configured so as to make the bottle free standing. The complex shapes are difficult to mold without stress. In addition, it has been found that stress introduced into the bottles in the blow molding process is accentuated when the bottles are returned to the bottler for reuse. Before refilling the returned bottles, the bottles are given a hot caustic wash and after a number of washes and refillings involving pressurizing the bottles and abrading the base area of the bottle on a conveyor, hazing or stress cracking develops almost exclusively on the exterior surfaces of the bottle. If severe enough, the stress cracking can result in loss of pressure and premature rupturing of the bottles.
It has been found that this stress cracking can be significantly minimized by applying to the exterior surface of the thermoplastic bottles a protective layer of a polyurethane.